Organosolv pulping and recovery process

ABSTRACT

A PROCESS IS PROVIDED FOR PULPING SUBDIVIDED FIBROUS PLANT MATERIAL, USING AS PULPING AGENT A MIXTURE OF WATER AND A WATER MISCIBLE, VOLATILE ORGANIC SOLVENT FOR EXAMPLE ETHANOL. THE DIGESTION IS CARRIED OUT IN A COUNTERCURRENT MANNER, THE SUBDIVIDED FIBROUS PLANT MATERIAL BEING IMMERSED IN THE PULPING LIQUOR AT AN ELEVATED TEMPERATURE AND PRESSURE. THE SPENT COOKING LIQUOR IS RECOVERED FROM THE DIGESTER AND IS CONDUCTED AT APPROXIMATELY THE PULPING CONDITIONS OF TEMPERATURE AND PRESSURE INTO A MULTISTAGE FLASH EVAPORATOR FROM WHICH THE ORGANIC SOLVENT IS RECOVERED CONTINUOUSLY AND CAN BE RECYCLED FOR MAKE-UP OF THE PULPING LIQUOR. THE MULTISTAGE FLASH EVAPORATION ALSO PERMITS RECOVERY OF LIGNIN AND WATER SOLUBLE COMPONENTS CONTAINING SUGARS, OLIGOSACCHARIDES AND ORGANIC ACIDS.

June 15, 1971 T. N. KLEINERT 3,535,194

ORGANOSOLV PULPING AND RECOVERY PROCESS Filed July 29, 1968 INVI'IN'I'UR THeodo N. KLEINERI .1 TIN/(N12. Y

United States Patent 3,585,104 ORGANOSOLV PULPING AND RECOVERY PROCESS Theodor N. Kleinert, 120 Embleton Crescent, Pointe Claire, Quebec, Canada Filed July 29, 1968, Ser. No. 748,320 Int. Cl. D21c 3/24 U.S. Cl. 162-17 8 Claims ABSTRACT OF THE DISCLOSURE A process is provided for pulping subdivided fibrous plant material, using as pulping agent a mixture of water and a water miscible, volatile organic solvent for example ethanol. The digestion is carried out in a countercurrent manner, the subdivided fibrous plant material being immersed in the pulping liquor at an elevated temperature and pressure. The spent cooking liquor is recovered from the digester and is conducted at approximately the pulping conditions of temperature and pressure into a multistage flash evaporator from which the organic solvent is recovered continuously and can be recycled for make-up of the pulping liquor. The multistage flash evaporation also permits recovery of lignin and water soluble components containing sugars, oligosaccharides and organic acids.

This invention relates to an organosolv pulping and recovery process and to an apparatus for carrying out said process.

More particularly, this invention relates to a process and an apparatus for subjecting to pulping subdivisions of various fibrous plant materials, such as wood, bamboo, bagasse, cereal straws, etc. at an elevated temperature, using aqueous mixtures of water-miscible, volatile organic solvents as pulping agents and recovering not only the cellulosic pulp and the volatile organic components of the pulping agent, but also the noncellulosic components of the fibrous plant materials solubilized during the pulping.

In most of the known chemical pulping processes, aqueous solutions of inorganic chemicals, such as the various sulfites, sodium hydroxide, mixtures of sodium hydroxide and sodium sulfide, are used as delignifying agents. For the so called hydrotropic pulping, sulfonates of various aromatic compounds, for instance, xylene sulfonate, have been suggested as pulping agents, exhibiting only negligible volatility. In all these pulping techniques, penetration of the pulping chemicals into the subdivided plant materials prior to heating to maximum reaction temperature is a prime factor.

In contrast to these conventional pulping processes, the process of the present invention, because of the volatility of the pulping agents, results in rapid and uniform penetration without the necessity of preimpregn-ation when the subdivided fibrous plant material is directly contacted with the liquor at maximum pulping temperature and pressure. Thus, the process of the present invention has the advantage that a separate stage of liquor impregnation of the fibrous plant material prior to pulping can be eliminated.

It is an object of the present invention to provide a new, superior and economical pulping process.

Other objects and advantages of the present invention will become apparent from the description to follow.

In accordance with the present invention, pulping is carried out at relatively mild conditions of liquor pH below and above the neutral point and the pulping liquor containing dissolved non-cellulosi components of the fibrous plant material is separated from the pulp dur- 3,585,104 Patented June 15, 1971 ing or soon after the pulping reaction without cooling ice this liquor i.e. at approximately the pulping conditions.

It was found that by keeping the liquor pH between about 4 and 10, viscosity and inherent strength properties of the pulped cellulosic fibres are preserved to a large extent. On the other hand, it was found that by separating the spent pulping liquor containing larger amounts of the non-cellulosic components dissolved from the fibrous plant material, from the pulp produced during or soon after the pulping reaction without substantial cooling, retention and/or reprecipitation of the said noncellulosic components, especially lignin, are greatly reduced. In this way, the purity of the pulp is improved, compared to cooling of the pulp in presence of this highly contaminated liquor.

In a tangible embodiment of the present invention, pulping of a fibrous plant material is carried out continuously with flow of the pulping liquor countercurrently to the flow of the subdivided plant material, for example wood chips, and the organic components of the spent pulping agent are recovered by multistage flash evaporation.

It was found that aqueous mixtures of the lower aliphatic alcohols, such as methanol, ethanol, propanol, and aqueous mixtures of the lower aliphatic ketones, such as acetone, or aqueous mixtures containing both lower aliphatic alcohols and lower aliphatic ketones are appropriate pulping agents according to the invention. Preferential pulping agents are aqueous mixtures of ethanol in the range between about 20% and ethanol by weight.

Furthermore, it was found that in order to shorten pulping time, the elevated temperature used during the pulping reactions should be between about C. and about 200 C., preferably between and 195 C. with the reaction time at a chosen temperature varying from one or more hours at the lowest temperatures to a few minutes at the highest temperatures.

In earlier work (U.S. Pat. 1,856,567), the inventor has shown that at temperatures between about 180 C. and 210 C. aqueous mixtures of the lower aliphatic alcohols, such as methanol, or ethanol in the medium concentration range between about 20% and 75 by weight alcohol content have a stronger delignifying eflect upon fibrous plant materials than the water-free alcohols. No noticeable amounts of the alcohols were found to be chemically consumed during the pulping reactions. These findings were based upon batch cooking with slow rise to temperature and using cooking times of several hours. The inventor has now elucidated some of the basic mechanisms underlying pulping of fibrous plant materials in aqueous ethanol (Thermal Pulping in Alcohol-Water Mixtures, T. N. Kleinert, Holzforschung und Holzverwertung 19, No. 4, 60-65 (August 1967)). It has now been found that, with rapid heating to maximum reaction temperature of the subdivided plant material submerged in the liquor, delignification is enhanced, and lignin solubilization takes place approximating an overall reaction pattern of apparently first order.

Thus, it is an advantage of the invention that pulping is carried out in the aqueous mixture containing the vola tile organic solvent with immediately reacting the subdivided fibrous plant material with the pulping agent at maximum pulping temperature and the corresponding pressure without requiring immediate previous impregnation of the plant material with the liquor. However, a brief pre-steaming of the subdivided fibrous plant material, as commonly practised in conventional pulping, for instance, ten minutes steaming at 1.05-2.00 kg./cm. steam pressure, is advisable not only in order to remove air from the plant material, but also to increase the cellwall permeability and thus to promote leaching out of the non-cellulosic portions from the pulps solubilized during pulping.

It was also found that when pulping is carried out in a continuous countercurrent flow system with continuous removal of the spent liquor for continuous recovery of the volatile organic components of the pulping agent without preceding cooling of the liquor, undesired secondary reactions, especially those which counteract delignification and contribute to lignin retention, are reduced, and thus, pulps of higher purity are produced.

Furthermore, it was found that the minor amounts of lignin which may be reprecipitated from the liquor onto the pulp fibres or still be present in a solubilized form within the cell walls can be easily removed by hot washing the pulp at atmospheric pressure with dilute alkali, for instance, an aqueous sodium hydroxide solution of 3-6 g. NaOH per liter, at 90100 C. temperature. When carrying out this washing in a closed system, this pulp purification can be combined with recovering the organic component of the pulping agent carried over with the pulp, by subjecting the alkaline liquor withdrawn from washing to a fractionated distillation and, if desired, recycling the recovered aqueous organic component into the pulping system.

In accordance with a preferred embodiment of the present invention, the spent pulping liquor is subjected to multistage flash evaporation, resulting in the recovery of aqueous condensates of the volatile organic components of the pulping agent for re-use.

In this multistage flash evaporation, in a known manner, the vapors formed are used as heating media in successive bodies before being withdrawn as condensates. This is achieved by introducing the spent pulping liquor into the first body of a multistage flash evaporator having a pressure which, although being the highest in the evaporator system, is appreciably lower than the digester pressure. As a result, because of the pressure differential between inand outgoing liquor, partial evaporation is caused by the heat content of the liquor with formation of vapor richer in said volatile organic component of the pulping agent than the remaining liquor portions with concurrent cooling of these liquor portions prior to entering the next body of lower pressure. Additional heat of evaporation in the second and the following bodies is supplied by the condensation of the vapors formed in the preceding body. In a known manner, this multistage flash evapo ration is operated by means of the pressure differentials between successive bodies of the evaporator while the remaining liquor portions flow from body to body and the vapors formed in each body are used for heating the next body while undergoing condensation with continuously withdrawing and recovering the enriched condensates for further use.

For the special case when aqueous ethanol is used as a pulping agent, the inventor has carefully studied the vaporphase liquid-phase equilibria of aqueous ethanol of varying concentration in the temperature range between 120 C. and 180 C. (The Liquid-Phase Vapor-Phase Equilibria of Ethanol-Water Mixtures at Temperatures Between 120 C. and 180 C., T. Kleinert, special issue No. 2, zu den Zeitschriften des Vereins Deutscher Chemiker, Verlag Chemie, Berlin, 1933). Based upon these experimental results, multistage flash evaporation for recovery of aqueous ethanol enriched in ethanol concentration from the spent pulping liquor can be calculated.

Thus, enriched aqueous ethanol fractions are recovered from ethanol-containing spent pulping liquors by multi stage flash evaporation.

Furthermore, it was found that during the distilling off of the volatile organic components of the pulping agent from the spent liquor for recovery and reuse, when reaching low concentrations of this volatile organic component, such as ethanol, the residual liquor portions separate into two layers of appreciably different specific gravity. The upper layer is an aqueous relatively light colored solution containing the water-soluble products derived from the fibrous plant material by the pulping reactions, such as monomer sugars, oligosaccharides, organic acids etc. The lower layer is a dark-brown, quasi molten flowing mass of greater specific gravity, mainly consisting of plasticized lignin substances. Based upon this finding and the appreciable differences of the specific gravity, the aqueous layer can be easily separated by known means from the layer of the highly plasticized, quasi molten lignin substances.

These findings permit to combine the recovery of the volatile organic components of the pulping agent from the spent pulping liquor with the separate recovery of the water-soluble derivatives of the fibrous plant materials on the one hand and of the related water-insoluble lignin substances on the other.

These lignin substances are highly plasticized and exhibit pronounced chemical reactivity. They can be used as raw materials for chemical conversion such as hydrogenation, oxidation, sulfonation, and other chemical reactions of the lignin, and also for condensation reactions, for instance, in the production of plastic materials.

On the other hand, the monomer sugars contained in the aqueous layer or formed by hydrolysis from the oligosaccharides also present in the aqueous solution, can be used in fermentation reactions, for instance to produce ethanol, or in protein production by microbiological processes. Acetic acid present in the aqueous layer, especially in hardwood pulping, can be recovered by known methods such as solvent extraction with appropriate water nonmiscible solvents, such as ethyl ether, isopropyl ether, butyl acetate, amyl acetate and others.

Therefore, the process of the present invention allows the separate recovery for further use of these two groups of substances derived from the fibrous plant materials, namely, the water-soluble products on the one hand, and the bulk of the plasticized lignin substances on the other.

Although the new process is preferably designed for continuous operation, it can also be carried out in a series of batch digesters with liquor flow from one pressure vessel to the next one in a quasi continuous countercurrent flow operation. In such case, for emptying and refilling at the end of the digestion period, each digester must be taken out of operation, while continuing the liquor flow in the remaining digesters. However, as will be apparent to persons skilled in the art, the multistage flash evaporator is nevertheless one of continuous operation.

The new process is practically odorless, and by recovering the organic components of the fibrous plant materials solubilized during the pulping reactions, it eliminates also stream pollution.

Thus, the pulping and recovery process of the present invention eliminates the problem of air pollution and also greatly reduces the problem of water and stream pollution.

The new process can be used for the pulping of various fibrous plant materials, especially of various wood species. In particular, it can be used for the pulping of mixtures of very different hardwoods. Since the volatile organic pulping agents penetrate easily and uniformly into all kinds of chipped wood, the new process can also be used for the pulping of hardwoods of high density.

Because of the mild pH conditions preferentially between pH 4 and pH 10, used during pulping and the relatively short pulping times, the viscosities and strength properties of the pulps produced are relatively high.

The new process can be used in the whole range of pulps produced from high-yield pulps of relatively high lignin content to low-lignin bleachable pulps.

In a preferred embodiment thereof the present inventional also comprises an apparatus for carrying out the process described, comprising a vertical elongated cylindrical digester, means for conducting subdivided fibrous plant material into said digester, means for conducting to said digester a pulping liquor comprising an aqueous volatile organic component, means for contacting said liquor and said subdivided fibrous plant material in said digester countercurrently, a pulping zone and a cooling zone in said digester, means for withdrawing from the cooling zone said pulping liquor and heating said pulping liquor to maximum pulping temperature and re-introducing the said heated liquor into the pulping zone of the digester, means for recovering digested wood chips, a multi-stage flash evaporator, means for withdrawing the spent cooking liquor from the pulping zone and for feeding said liquor at the operating temperature and pressure of said digester into said multi-stage flash evaporator, means for recovering from said multi-stage evaporator a condensate enriched in said aqueous volatile organic component and a residual liquid low in said aqueous volatile organic component, means for recirculating said condensate enriched in said aqueous volatile organic component to said digester, means for allowing said residual liquid low in said aqueous volatile organic component to separate by specific gravity and means for separately recovering the two phases thus obtained.

In a further embodiment thereof the present invention comprises an apparatus for carrying out the process described, said apparatus comprising a vertical elongated cylindrical digester having means for continuously introducing thereinto subdivided fibrous plant material against the digester pressure at the top of the digester, means for continuously withdrawing the pulp produced in a partly Washed and cooled state at the bottom of the digester, this digester having strainer rings for continuously introducing and withdrawing pulping liquor to provide liquor upflow in two temperature stages With circulation and heating means for the liquor between these two stages, the uppermost strainer ring being used to withdraw spent pulping liquor for recovery and also to provide a constant liquor level in the digester with a small vapor phase above connected to release means, the outlet of the digester for the spent pulping liquor being connected to a multistage flash evaporator, means for Withdrawing the condensates from said evaporator, the last body of said evaporator discharging the remaining liquor into a separation tank having withdrawing means for separately recovering the water insoluble highly plasticized lignin portions of relatively high specific gravity and the solution of the water soluble components of low specific gravity, the vapor phase of the last body being connected to a fractionating condenser column, having means for continuously releasing separately, permanent gases, water, and the aqueous condensate containing volatile organic solvents, means for adjusting concentration and temperature of said condensates and means for recycling said condensates into the bottom strainer of the digester.

The new organosolv pulping and recovery process of the present invention will now be further described in more detail with reference to the following non-limiting example and to the accompanying drawing the single figure of which represents a schematic illustration thereof.

EXAMPLE Commercial aspen chips of about inch length are continuously introduced into chip bin 1 from which they enter low-pressure steaming vessel 3 through low-pressure rotary valve 2. While being transported through vessel 3 by screw conveyor 4, the chips are treated by a countercurrent flow of low-pressure steam (1.2 kg./cm. supplied to vessel 3 through pipe 5 and withdrawn through exhaust pipe 6 connected to rotary valve 2 and fractionating condenser column 23. The spent steam contains small amounts of ethanol leaking from the digester through high-pressure rotary valve 7, and after being withdrawn through exhaust pipe 6, it is subjsected to fractionated condensation first in condenser column 23 and then in condenser column 21 connected by pipe 20.

The chips, steam-treated in vessel 3, are -fed continuously through high-pressure rotary valve 7 into vertical, cylindrical digester 8. Digester '8 is equipped with a series of strainer rings 9, 10, 11, 12, namely, strainer rings 9 and 10 for introducing liquor and strainer rings 11 and 12, for withdrawing liquor from the digester.

Freshly recovered aqueous ethanol of about 45.8% ethanol content having a pH of 4 and a temperature of C. and additional re-cycled liquor of similar composition and temperature, recovered from the partially washed pulp in the screw press 13 is forced into digester 8 by means of the rotary pump 28 via pipes 67 and 29 and strainer ring 9. A major portion of this liquor mixture flows in the digester upwards countercurrently to the pulp while partly washing and cooling it, and another portion of the liquor mixture flows concurrently with the cooled pulp through conduit 30 into the screw press 13, where the pressed pulp is discharged from the pressurized system of the digester. The liquor portions of relatively low temperature which, after being introduced into the digester through strainer ring 9, flow upwards countercurrently to the pulp are withdrawn through strainer ring 11. This liquor is re-cycled from strainer ring 11 to strainer ring 10 by rotary pump 15 via pipe 14, heater 16 and pipe 17. In this manner, the liquor is heated up to maximum pulping temperature C.) before being re-introduced into the digester. While the liquor again flows upwards in the digester between strainer rings 10 and 12 countercurrently to the plant material, pulping takes place at maximum temperature 180 C.) in liquid phase with solubilization of major portions of the noncellulosic components. Time of exposure of the plant material to the liquor at maximum temperature is about 30 minutes. Through strainer ring 12, the spent pulping liquor containing about 10% by weight of solubilized non-cellulosic portions of the Wood is continuously withdrawn from the digester via pipe 31 directly into body 32, the first body of the five-stage flash evaporator 66 consisting of bodies 32, 33, 34, 35, 36 and fractionating column 37 which operates at atmospheric pressure. By means of strainer ring 12, a constant level of the pulping liquor is maintained in digester 8 and a small vapor phase is formed above the liquor level in the top section of the digester.

Chips presteamed in vessel 3, containing appreciable amounts of water (50% and even more) enter the digester through the high-pressure rotary valve 7 and pile up for a brief period of time on top of the chip column. Thus, these chips are briefly exposed to a vapor phase considerably richer in ethanol content than the corresponding hquor. In this way, the chips are not only rapidly heated to pulping temperature but concurrently are also impregnated with aqueous ethanol according to the ethanolwater vapour-phase liquid-phase equilibrium at pulping temperature. Thus, the chips when subsequently submerged mto the liquor are at cooking conditions, so that pulping starts immediately.

The vapor phase in the top section of the digester is connected via pipe 18 and release valve 19 for continuously releasing sufi'icient amounts of the vapor mixture in order to remove permanent gases such as air and carbon dioxide accumulating in the vapor phase during pulping. In this way also, water portions are removed from the digesting system. Valve 19 is connected via pipe 20 to the fractionating condenser column 21 for recovery of aqueous ethanol from said withdrawn vapor. In the fractionating condenser column 21 also, the ethanol-water vapors partially enriched in ethanol in condenser column 23 are further subjected to fractionated condensation. The total of the aqueous ethanol recovered in fractionating condenser column 21 flows via pipe 65 by means of the rotary pump 22 into the liquor make-up tank 27 for re-use. Non-condensable gases (air and carbon dioxide) are released from the fractionating condenser column 21 through pipe 64. The water is withdrawn from fractionating condenser column 21 through pipe 62 together with some condensed volatiles derived from the Wood in some cases, such as terpenes.

The partly washed and cooled pulp together with portions of the liquor are fed by the digester pressure through conduit into the screw press 13. Liquor squeezed out from the pulp in this screw press is re-cycled via pipe 67 by rotary pump 28 and via pipe 29 into strainer ring 9 and enters the digester together with fresh liquor from liquor make-up tank 27 also supplied by rotary pump 28 via pipe 29. For further washing of the pulp and recovery of residual ethanol, the pulp is treated in washer 24 in a countercurrent flow of weak aqueous sodium hydroxide (3-6 g. NaOH per liter) solution at 90-100" C. entering through pipe 25 and being withdrawn through pipe 26 for recovering the ethanol portions by fractionated distillation. The pulp is recovered for further treatment by the screw press 60 and the liquor thus removed is re-cycled into washer 24 via pipes 68 and 25.

The properties of the Washed and screened pulp obtained are listed in Table I.

TABLE I Pulp yield calculated on CD. wood54.0%

Lignin content of the pulp-1.9%

0.5% CED viscosity of the pulp determined by TAPPI Standard Method T230 sm50, calculated on the alphacellulose portions, free of lignin50 cps.

Unbeaten pulp at 540 CSF:

Tear factor-72 Breaking length7260 m. Pulp beaten to 300 CSF:

Tear factor-75.

Breaking length12,000 m.

The spent pulping liquor withdrawn from the digester through strainer ring 12 at digester temperature (180 C.) and pressure (17.4 kg.-cm. contains per 100 kg., about 41.6 kg. ethanol, 48.4 kg. Water and 10 kg. noncellulosic solids solubilized from the wood during pulping. This liquor enters via pipe 31 into the first body 32 of the five-stage fiash evaporator 66, consisting of the bodies 32, 33, 34, 35, 36 and fractionation column 37, and operating at falling pressures and temperatures. While Residual liquor (1:51.)

From separation tank 38 the plasticized lignin is withdrawn through valve 39 whereas the aqueous phase is carried through pipe 73 into the fractionating column 40 for stripping off the ethanol portions. These portions are recovered in condenser 41 and are ultimately withdrawn through pipe 55 and re-cycled into the liquor make-up tank 27 via pipes 55 and 57 by means of rotary pump 58. The residual aqueous solution containing sugars and other components is withdrawn through valve 59.

Pipes 42, 43, 44 and 45 connect the vapor phase of bodies 32, 33, 34 and 35 to the condenser-heater elements of the subsequent bodies 33, 34, 35 and 36, respectively. Pipe 46 connects the vapor phase of body 36 to the fractionating condenser column 37. Pipes 47, 48, 49 and 50 carry the residual liquor from the first body 32 to the subsequent bodies 33, 34, 35 and 36. Pipes 51, 52, 53, 54, 55, 56 and 57 feed the condensates of aqueous ethanol recovered from the bodies 33, 34, 35, 36 and from the fractionating column 37 and condenser 41 via pipe 57 into the liquor make-up tank 27 by means of the rotary pump 58. Pipes 51, 52, 53 and 54 are equipped with cooling devices for the condensates (not shown in the drawing) to reduce the temperature of the corresponding condensates to a temperature level not substantially higher than about 100 C. Valves 61, 62 and 63 indicate the water outlets from the fractionating condenser columns 37, 21 and 23 respectively. Liquor make-up tank 27 is equipped with known means (not shown in the drawing) for cooling or heating the recovered mixture of the pulping agent, and also with other means (not shown) for adjusting the resulting mixture to the desired level of concentration and temperature for re-use in pulping and introduction into digester by means of rotary pump 28 via pipe 29 and strainer ring 9.

Table II shows the approximate amounts and composition of the residual liquor leaving the bodies 32, 33, 34, 35 and 36 of the five-stage flash evaporator 66, and those of the ethanol-water vapors formed in these bodies, and also the corresponding vapor pressures, assuming that all the vapors have been condensed.

TABLE 11 Ethanol-water vapor Ethanol Water Total Ethanol Pressure Body Ethanol Water Total (kg) (kg) (kg) (percent) (kg/emfi) 32 33. 3 44. 8 78. 1 8. 3 3. G 11. 9 70 6. 5 33 21. 8 39. 4 61. 2 11. 5 5. 2 16. 7 69 3. 5 34 10. 1 33. 4 43. 5 11. 6 (i. 3 17. 9 2. 3 35 2. 6 25. 2 27. 8 7. 6 8. 2 15. 8 48. 1 1. 4 36 (l. 6 17. 2 17. 8 2. 0 8. 0 10. 0 20. 0 1. 1

the residual liquor flows from body 32 through bodies 33, 34, 35 into body 36 partial evaporation of aqueous ethanol of relatively high ethanol content takes place in each body partly by the differences of the heat contents of the liquor in the various bodies, and partly by the condensation heats of the vapors from each preceding body used as heating medium in the condenser-heater elements resulting in condensation of the vapors. From body 36 the residual liquor is withdrawn into the separation tank 38 where the water-insoluble, molten phase of highly plasticized lignin of relatively high specific gravity separates from the aqueous layer of lower specific gravity and accumulates in the lower part of tank 38. The vapors formed in body 36 undergo fractionated condensation in the fractionating condenser column 37 which operates at atmospheric pressure.

Fractions of non-condensed vapor which may contain some permanent gases are carried through connecting pipes '69, 70 and 71 into the condenser-heater elements of the next body. Vapor flow in these pipes 69, 70 and 71 is controlled by valves not shown in the drawing. Pipe 72 conducts residual vapor and any permanent gas present from the condenser-heater element of body 36 to the fractionating column 37, from which the permanent gases are expelled through outlet 74.

The residual liquor (27.8 kg.) withdrawn from body 36 contains about 10 kg. non-cellulosic solids solubilized from the wood during pulping. In separation tank 38 this liquor separates easily into a non-aqueous, dark-brown, quasi molten phase of highly plasticized lignin (about 4.5- 5 kg. calculated on kg. of the initial spent pulping liquor) of about 1.5 specific gravity and an aqueous phase (22.823.3 kg.) of about 1.1-1.2 specific gravity containing about 5-5 .5 kg. monomer sugars, oligosaccharides and also acetic acid derived from the Wood. From this aqueous fraction withdrawn from the separation tank 38 into the fractionating column 40 residual ethanol and methanol split off from the wood during pulping can be recovered in vessel 41 and withdrawn therefrom through pipe 55 for further use. The aqueous fraction stripped of ethanol and methanol in this way is withdrawn from column 40 through valve 59. This fraction contains up to about 2% acetic acid calculated on the initial wood which can be recovered by extraction with isopropyl ether or butyl acetate.

The aqueous solution of the wood sugars and oligosaccharides, preferably after hydrolysis of the oligosaccharides, can be used as a nutrient for microorganisms in fermentation processes or for biological production of proteins. After removal of the water, the solids of the residue can be used as a fodder for stall-feeding cattle, pigs, etc.

The highly plasticized lignin phase, after recovering the small amounts of ethanol and methanol retained, preferably by steam stripping, can be used directly in the production of plastic materials or as raw material in the organic industry for chemical conversion such as hydrogenation, oxidation, sulfonation, etc.

The above example describes the flash-evaporation recovery of aqueous ethanol as representative of the aqueous volatile organic pulping agents used according to the present invention. However, as will be apparent to persons skilled in the art of flash evaporation, the first body of the flash evaporator can be equipped with heating elements for partly steam heating the liquor withdrawn from the digester. In this case, an increased amount of heat is available in the first body for liquor evaporation than without this additional heating. This first body can then be of reduced size, and also the number of bodies can be reduced from five to four.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

I claim:

1. A continuous method for digesting subdivided fibrous plant material in a digester at an elevated digesting pressure and at an elevated digesting temperature without preimpregnation of pulping agent which consists essentially of (a) feeding said subdivided fibrous plant material to an inlet of said digester and moving said fibrous plant material through the digester to a fibrous plant material outlet remote from said inlet;

(b) introducing a liquid pulping agent into said digester at a point intermediate the fibrous plant material inlet and the fibrous plant material outlet, said pulping agent being at a temperature corresponding essentially to said digesting temperature, and being an aqueous mixture of a member selected from the group consisting of a lower aliphatic alcohol, a lower aliphatic ketone and their mixtures and containing about 20-75 weight percent of said member;

(c) flowing said pulping agent in countercurrent contact with said fibrous plant material, heating said fibrous plant material to substantially said elevated digesting temperature substantially immediately on its being fed into said digester, and dissolving noncellulosic water-soluble components of said fibrous plant material in said pulping agent on its being contacted with said pulping agent;

(d) withdrawing pulping agent containing said noncellulosic components from said digester at a point adjacent said subdivided fibrous plant material inlet, said withdrawn pulping agent having a temperature corresponding substantially to said elevated digesting temperature so that no appreciable cooling of the Withdraw-n pluping agent occurs; and

(e) withdrawing digested fibrous plant material from said digester through said fibrous plant material outlet.

2. The continuous method of claim 1 which includes passing said pulping agent withdrawn from -(d) above through a plurality of successive flash expansion stages, thereby producing a condensate enriched in the alcohol or ketone component of the pulping agent and a residue comprising water-insoluble plasticized lignin and said noncellulosic water-soluble components.

3. The continuous method of claim 2 which includes separating said residue into a water-insoluble plasticized lignin fraction and a non-cellulosic Water-soluble component fraction.

4. The continuous method of claim 1 wherein said elevated digesting temperature ranges between about 200" C.

5. The continuous method of claim 4 wherein the temperature ranges between -495 C.

6. The continuous method of claim 1 wherein said pulping agent is an aqueous ethanol mixture.

7. The continuous method of claim 1 wherein the pulping agent has a pH ranging between about 4-l0u 8. The continuous method of claim 1 wherein the fibrous plant material fed to said digester in (a) moves downwardly therethrough and the liquid pulping agent introduced into said digester in (b) for countercurrent contact in (c) with said fibrous plant material flows upwardly therethrough.

References Cited UNITED STATES PATENTS 1,856,567 5/1932 Kleinert et a1. 162--45 3,097,987 7/1963 Slowman l62l9 3,176,756 4/1965 Dukelow 15947WL 3,294,623 12/ 1966 Brinkley et al. l62l9 3,428,107 2/1969 Backteman 159-47WL S. LEON BASHORE, Primary Examiner T. G. SCAVONE, Assistant Examiner US. Cl. X.R. 159-47; 162-32, 77 

